1. Field of the Invention
The present invention relates to a layered formation including a photocatalyst and a making method thereof, a printing plate able to be reused and a making method thereof, and a printing press.
2. Description of the Related Art
In recent years, digitalization of printing process has become progressing in the art. This technology involves creation of images and manuscripts in digitized form on a personal computer or reading images on a scanner and directly makes a printing plate based on the digital data thus obtained. This makes it possible to save labor in the printing processes and also to conduct high-precision printing with ease.
As printing plates, so-called PS plates (presensitized plates) have been commonly used to date. A PS plate includes a hydrophilic non-image area made of anodized aluminum and has one or more hydrophobic image areas formed by hardening a photosensitive resin on the surface of the anodized aluminum. Making of a printing plate with such a PS plate requires plural steps and hence, is time-consuming and costly. It is, therefore, difficult to reduce the time and the cost required for printing. Especially in short run printing, the requirement for the plural steps is a cause of increased costs. Further, use of a PS plate requires a development step using an alkaline developer, which has raised serious problems both with the amount of labor required and also with environmental pollution caused by treatment of developer waste.
Further, it is a common practice to expose a PS plate on whose surface which functions as a printing plate, a film with an original picture image formed thereon with perforated holes is closely attached. The above manner becomes problematic in making the printing plate directly from digital data and in promoting a digitized printing process. Moreover, after a print job, it is necessary to replace the printing plate with another one in order to conduct printing of a next print job so that used printing plates have been thrown away.
To solve the above-described problems of PS plates, processes have been proposed to meet the digitization of printing processes while making it possible to omit the development step, and some of such processes have come into commercial use. For example, Japanese Patent Application Laid-Open (KOKAI) Publication No. SHO 63-102936 discloses a process of making a plate which comprises the steps of: applying a jet-form ink containing a photosensitive resin, as an ink for a liquid ink-jet printer, onto the surface of a printing plate; and hardening an image area by irradiation with light. Japanese Patent Application Laid-Open (KOKAI) Publication No. HEI 11-254633, on the other hand, discloses a process for making a color offset printing plate by an ink-jet head through which a solid ink is jetted.
Also included in known processes are a process for making a printing plate, which comprises the step of writing with a laser beam an image on a printing plate, which is made of a PET (polyethylene terephthalate) film on which a laser absorbing layer such as carbon black covered with a silicone resin layer is formed, to cause the laser absorbing layer to evolve heat, which ablates off the silicone resin layer by the heat; and another process for making a printing plate, which comprises the step of coating an aluminum plate with a lipophilic laser absorbing layer, coating a hydrophilic layer on the laser absorbing layer, and then ablating off the hydrophilic layer with a laser beam as in the above-described process.
In addition, a process has also been proposed for the making of a printing plate made of a hydrophilic polymer by exposing the hydrophilic polymer imagewise such that the hydrophilic polymer becomes lipophilic.
Further, a process of directly forming an image on a PS plate with light from a digital data is also proposed. For this purpose, an imaging unit utilizes blue laser having a wavelength of 402 nm, and a so-called CTP (Computer To Plate) device including a micromirror and a UV (ultraviolet) lamp have been put onto the market.
But, since each of the above-mentioned processes cannot continue to perform an ensuing print job (printing an ensuing print pattern) until a printing plate is replaced by another one subsequent to completion of a print job, they are not different from the conventional art in that the printing plate is thrown away after its one-time use even if it is possible to make printing plates directly from digital data.
On the other hand, a technique including regeneration of a printing plate is disclosed. For example, Japanese Patent Application Laid-Open (KOKAI) Publication No. HEI 10-250027 refers to a latent image block copy making use of a titanium dioxide photocatalyst, a making process of the latent image block, and a printing press having the latent image block. Japanese Patent Application Laid-Open (KOKAI) Publication No. HEI 11-147360 also discloses an offset printing process by a printing plate making use of a photocatalyst. Each of these disclosures forms an image using light (practically, ultraviolet light) to activate the photocatalyst and regenerates a printing plate by hydrophobization of the photocatalyst caused by heat treatment. Further, Japanese Patent Application Laid-Open (KOKAI) Publication No. HEI 11-105234 discloses a making process for a lithographic printing plate, which comprises the step of hydrophilizing a photocatalyst with activating light, i.e., ultraviolet rays, and then forming an image area by a heat-mode recording.
As disclosed in the paper (pages 124-125) entitled “Study on Behavior of Photoinduced Hydrophilization Associated with Structural Change in Titanium dioxide Surface (by Minabe, et al.) distributed at the Fifth Symposium on “Recent Developments of Photocatalytic Reactions” of the Photo Functionalized Materials Society in 1998, Prof Fujishima, Prof Hashimoto, et al. of the Research Center for Advanced Science and Technology, The University of Tokyo, have confirmed hydrophilization of a titanium dioxide photocatalyst by heat treatment. According to the description in the above paper, the processes, or heat treatment, disclosed in the laid-open patent applications referred to in the above, cannot hydrophobize photocatalyst thereby regenerate a printing plate, so that neither recycling nor newly preparing a printing plate is available.
With the foregoing problems in view, the Inventors have been enthusiastically researching to develop a printing plate on which an image is rapidly formed using activating light having energy higher than a band-gap energy of the photocatalyst included in the printing plate, which light is emitted from an imaging unit, and which can be regenerated for future reuse after the formed image is printed, and methods for making and regenerating such a printing plate.
As a result of the research, it was found that the surface of a printing plate needs to be hydrophilized when an image is formed on the printing plate or the image is deleted from the printing plate for regeneration. It is an important object of such processes to execute hydrophilization in a shorter time with a weaker light.
As to photo-induced hydrophilization (hereinafter simply called hydrophilization), the paper (pages 44-45) entitled “Effects of TiO2/WO3 Thin Films on Hydrophilization properties” (by Irie, et al.) distributed at the Eighth Symposium on “Recent Developments of Photocatalytic Reactions” of the Photo Functionalized Materials Society in 2001 discloses a technique of sensitization of photocatalytic activity, especially photo-induced hydrophilization, by forming a thin film of TiO2 (titanium dioxide) on a thin film made of WO3 (tungsten trioxide).
The recent research estimates that positive holes relate to hydrophilization of titanium dioxide. Namely, those skilled in the art estimate that the sensitivity of hydrophilization is improved by making TiO2 and WO3 into a complex form so that movement of electrons generated in TiO2 by irradiation with activating light to WO3 improves charge separation (preventing an electron and a positive hole from re-combining) efficiency and by providing TiO2 with positive holes generated in WO3.
Application of such a technique to a printing plate sensitizes hydrophilization of a photocatalyst layer (in other words, reduces energy required for the hydrophilization) so that an image is formed on the printing plate with weaker light. It is therefore possible to shorten time to form an image on the printing plate and time to regenerate the printing plate. The formation and the regeneration should be however performed faster in order to speed up printing of an image.
Japanese Patent No. 2917525 discloses a method for hydrophilizing the surface of the substrate coated with a layer including a photocatalyst. More specifically, the surface of the layer including a photocatalyst supports a metal oxide complex serving as a solid acid and the surface is photocatalytically hydrophilized. The metal oxide complex is an oxide hyperacid, which photocatalytically hydrophilizes the surface of the substrate by increasing hydrogen-binding components (γsh) on the surface energy of solid/gas surface of the layer including the photocatalyst.
Public re-disclosed Patent No. WO97/23572 refers to a method for hydrophilzing the surface of the substrate coated with a layer including a photocatlayst. Supporting an oxide hyperacid on the surface of the layer including a photocatlayst increases hydrogen-binding components (γsh) on the surface energy of solid/gas surface of the layer including the photocatalyst so that the surface is hydrophilized by physically adhering water molecules to the surface of the layer.
But the disclosure of above-mentioned U.S. Pat. No. 2,917,525 does not mention influence on one of the important and generic properties of a photocatalyst, that is photocatalytic activity to decompose an organic compound, which influence is caused by supporting a metal oxide complex serving as an oxide hyperacid on the surface of the layer including a photocatlayst.
Since the technique described in the above re-publication WO97/23572 refers to preservation of high hydrophilicity by supporting an oxide hyperacid on the surface of the layer including a photocatlayst to allow water molecules to physically adhere to the surface of the layer, it is clear that the technique does not aim at improving a photocatalyst's property of decomposing an organic compound.
The Inventors have been enthusiastically researching to find factors required to improve a photocatalyst's properties of decomposing an organic compound and at the same time hydrophilzing the photocatalyst itself.